Unions

A union type represents a set of object types. It is very similar to an interface, except that there is no requirement for common fields between the specified types.

SDL

type TextContent {
  text: String!
}

type ImageContent {
  imageUrl: String!
  height: Int!
}

union PostContent = TextContent | ImageContent

Clients can query fields returning a union like the following.

GraphQL

{
  content {
    ... on TextContent {
      text
    }
    ... on ImageContent {
      imageUrl
    }
  }
}

Learn more about unions here.

Usage

Unions can be defined like the following.

We can use a marker interface (or an abstract class) to define object types as part of a union.

[UnionType("PostContent")]
public interface IPostContent
{
}

public class TextContent : IPostContent
{
    public string Text { get; set; }
}

public class ImageContent : IPostContent
{
    public string ImageUrl { get; set; }

    public int Height { get; set; }
}

public class Query
{
    public IPostContent GetContent()
    {
        // Omitted code for brevity
    }
}

builder.Services
    .AddGraphQLServer()
    .AddQueryType<Query>()
    .AddType<TextContent>()
    .AddType<ImageContent>();


C#
public class TextContent
{
    public string Text { get; set; }
}

public class TextContentType : ObjectType<TextContent>
{
}

public class ImageContent
{
    public string ImageUrl { get; set; }

    public int Height { get; set; }
}

public class ImageContentType : ObjectType<ImageContent>
{
}

public class PostContentType : UnionType
{
    protected override void Configure(IUnionTypeDescriptor descriptor)
    {
        descriptor.Name("PostContent");

        // The object types that belong to this union
        descriptor.Type<TextContentType>();
        descriptor.Type<ImageContentType>();
    }
}

public class Query
{
    public object GetContent()
    {
        // Omitted code for brevity
    }
}

public class QueryType : ObjectType<Query>
{
    protected override void Configure(IObjectTypeDescriptor<Query> descriptor)
    {
        descriptor
            .Field(f => f.GetContent(default))
            .Type<PostContentType>();
    }
}


Since the types are already registered within the union, we do not have to register them again in our Program class.
We can use a marker interface, as in the implementation-first approach, to type our union definition: UnionType<IMarkerInterface>

Last updated on February 17, 2026 by Michael Staib